Various vinyl plastisol compositions are known and used in various slush molding methods in various assemblies for performing those methods. Typically, an open hollow mold is filled with liquid plastisol or covered by dry plastisol and heat is applied to the mold surface and transferred to the liquid or dry plastisol to gel a layer of plastisol adjacent the mold surface. The thickness of the gelled layer depends upon the temperature to which the plastisol layer is subjected and the time the plastisol is maintained at that temperature. After the layer adjacent the mold surface is gelled, the ungelled plastisol is dumped or poured out of the mold.
In one well-known method, the mold temperature is maintained relatively low to prevent gelling as a very thin initial layer or coating of plastisol is applied to the mold to prevent bare spots and to prevent the entrapment of air bubbles, thereby providing a thin coating which strictly conforms to the mold configuration. Such a coating is applied by filling and emptying the mold. For example, automotive dashboard or instrument panel covers are made in this fashion and the molds frequently include details to define imitation stitching in the finished product as well as undercuts and other intricate detail. The very thin coating is first applied to conform to the mold and to prevent irregularities in the outer surface of the finished product. After this very thin coating is applied, the mold is again filled with additional plastisol and the entire mold is heated to gel the plastisol and increase the thickness of the finished product. After the desired thickness is attained, the mold is again dumped or emptied and thereafter the mold is subjected to additional heat for curing the product.
An example of such a method and an assembly for practicing same is shown in the U.S. Pat. No. 3,728,429 granted to Daniel E. Colby, Philip E. Rogers and Frederick J. Sliwinski on Apr. 17, 1973 and assigned to the assignee of the subject invention. That patent discloses such a method for slush molding articles wherein the mold is heated by the exterior surfaces thereof being exposed to impinging streams of hot gas and, after the product is finished, the mold is cooled by being subject to cooling water from water spray nozzles, after which the fused finished article is stripped from the mold. In addition, an endless conveyor moves a plurality of molds through various stations in the performance of the method. Although the method and assembly shown in this patent have proven to be very satisfactory, there are disadvantages such as the open flame adjacent the molds, which produces the hot gas for heating of the molds, and the use of water or liquid for cooling the molds and which can be incompatible with the plastisol. Further, the assembly, because of its long conveyor and multiple molds, is suited for long runs of a plastisol of a particular color but is not well suited for short runs or quick or efficient changeovers.
Other methods of heating in a slush molding process have been utilized in the prior art; for example, the molds may be moved through heating ovens as exemplified in the U.S. Pat. No. 3,002,230 granted to J. W. Stewart on Oct. 3, 1961. Alternatively, the molds may be subjected to induction heaters as exemplified in the U.S. Pat. No. 3,315,016 granted to John M. Wersosky and Donald A. Moore on Apr. 18, 1967 and assigned to the assignee of the subject invention. Another method for heating the mold is exemplified in U.S. Pat. No. 3,680,629 granted to Laurent R. Gaudreau and Floyd E. McDowell on Aug. 1, 1972 and assigned to the assignee of the subject invention. That patent teaches the heating of a mold by incorporating tubes in the mold and flowing a heated fluid such as steam through the tubes for heating the mold. It is also known in the slush molding art to heat the mold by such tubes for conveying liquid through the mold wherein there are multiple circuits of the tubes with each circuit having an inlet and an outlet, but with each circuit subjected to the same fluid medium, i.e., the same temperature.
U.S. Pat. No. 4,623,503 granted to Emmanuel Anestis and Frederick I. Wakefield on Nov. 18, 1986, with a common assignee, discloses an assembly controlling the thickness of an article made by the slush molding process or dry plastic casting process in a mold having first and second groups of gas passages defined by gas jet nozzles disposed adjacent the mold surface with dampers for controlling the temperature of respective first and second areas of the mold surface.
U.S. Pat. No. 4,621,995 granted to John M. Wersosky on Nov. 11, 1986, and assigned to the assignee of the subject invention, provides an improved valve assembly with two drive cylinder positioned valve plates for controlling the thickness of an article made by the slush molding process in a mold having first and second impingement jet distribution means and a gas heating and cooling system for controlling the temperature over respective first and second areas of the mold surface. The first group of impingement jet nozzles are disposed adjacent the first area of the mold surface where it is desired to increase the thickness of the finished article whereas the second group of impingement jets is disposed adjacent the second area of the mold surface where the material in the finished article is scrapped and/or will be trimmed and therefore the desired thickness is desirably less. The first and second groups of impingement jet nozzles are supplied with gas by regulating the cylinders to position the valve plates thereby to control the temperature of the first and second areas of the mold surface at a non-gelling temperature as plastisol is disposed over the mold surface to define a coating or layer. This very thin coating of plastisol is applied to be free of surface blemishes. Thereafter, additional plastisol is provided for increasing the thickness only over the first area of the mold surface. To accomplish this, the valve plates are positioned so that hot gas flows through first group of impingement jet nozzles to heat the first area of the mold surface to a higher temperature than the second area so as to gel the plastisol over the first area of the mold surface, while producing thinner thickness in the molded article at a second region which is scrap or trim area of the molded article.